Drilling fluid containing microspheres and use thereof

ABSTRACT

The present invention provides a drilling fluid composition and a method of drilling comprising a drilling fluid, which may be oil- or water-based, and a composite microsphere component. The invention drilling fluid composition advantageously reduces the density of a conventional fluid, reduces costs associated with pumping and overcomes problems associated with conventional gas-injection processes. The invention uses conventional drilling and pumping equipment, requires no sea-floor based pumps and may be easily pressure-controlled to maintain the pressure of the fluid.

This application is a continuation of U.S. Ser. No. 10/218,962, filedAug. 14, 2002, now pending, the disclosure of which is hereinincorporated by reference.

This invention relates to a novel drilling fluid composition and usethereof in drilling wells for petroleum and natural gas.

BACKGROUND

In oil drilling operations, a drilling fluid is circulated downwardlythrough a drill string to cool and lubricate the drill string, suspendthe cutting removed from the well bore and to keep out formation fluids.The drilling fluid containing the suspending cuttings are furthercirculated upwardly through the annulus between the drill string andwall of the well bore to the surface, where the cutting are separatedand the recycled drilling fluid is circulated down the bore. Drillingfluids, also known as drilling muds, may be oil- or water-based. Bothwater-based and oil-based drilling fluid systems are known. The moreeconomical water-based systems are used when practicable with oil-basedsystems being used where increased lubricity at the drilling head isdesirable or when traversing formations which would be adverselyaffected by a water-based system, such as water soluble shaleformations.

A conventional oil-based drilling fluid (mud) generally comprises an oilfluid vehicle, such as a diesel oil, emulsifying agents, such asalkaline soaps of fatty acids, wetting agents or surfactants, such asdodecylbenzene sulfonate, water, generally as a NaCl or CaCl₂ brine, anda viscosifying agent, such as an amine treated clay. Oil-base fluids mayhave an aromatic or aliphatic oil, or a mixture of oils, as thecontinuous phase. These oils may include diesel, mineral or synthetic(PAO, esters, ether) oil. They may be comprised entirely of oil or, morecommonly, may contain water ranging from 5% to upwards of 50-60%. In thelatter case, water becomes the internal phase, is emulsified into theoil as a heterogeneous fine dispersion, and the resulting system isreferred to as an oil-based or oil-invert emulsion fluid.

A water-based drilling fluid comprises a viscosifying agent, generally aclay such as a solid phase bentonite attapulgite or sepiolite, and awater fluid vehicle. In addition, salt or salt water can be added to thecomponents of the drilling fluid to prepare a salt water based drillingfluid. Numerous different additives to this drilling fluid are alsoemployed to control viscosity, yield point, gel strength (thixotropicproperties), pH, fluid loss, tolerance to contaminants such as salt andcalcium carbonate, lubricating properties, filter caking properties,cooling and heat transfer properties, and tolerance to inactive solidssuch as sand and silt or active native mud making clays such assmectites, illites, kaolinites, chlorites, etc. Clays are not usuallyused as the sole viscosifying agent and typically organic water-solublepolymers such as starch, carboxymethylcellulose, natural gums orsynthetic resins are used in conjunction with clays. These polymers alsoaid the clay component of the drilling fluid to serve as a filtrationaid to prevent or retard the drilling fluid from being lost into theformation.

A number of drilling fluid formulations have been described. Forexample, U.S. Pat. No. 3,726,850 discloses a lignin dispersing agent fordispersing clays, and the like. The lignin dispersing agent is reportedto have utility in both alkaline and acidic media. A relatively lowviscosity aqueous silicate solution is disclosed in U.S. Pat. No.3,746,109, and is reported to be particularly useful in drilling throughshale formations. U.S. Pat. No. 4,799,549 discloses a stable,gel-forming microemulsion comprising an aqueous solution of an alkalimetal silicate, a gelling reagent, and a surface-active agent(surfactant). This composition is reported to be useful for permanent orreversible plugging or clogging of subterranean formations. Also, U.S.Pat. No. 5,374,361 discloses a composition for cleaning out casedwellbores, and the like, using a fluid that includes a caustic alkylpolyglycoside surfactant formulation. This formulation is reported to bemore biodegradable than previous detergent systems. A further additiveencountered in aqueous drilling fluids is a metal compound, such as thatdescribed in U.S. Pat. No. 5,399,548, or a derivative of a metalcompound such as a hydroxy-aluminum compound provided in a polymer, suchas disclosed in U.S. Pat. No. 4,045,357. U.S. Pat. No. 5,333,698 alsodiscloses a drilling fluid additive in combination with a whitenon-toxic mineral oil.

Although oil- and water-based drilling fluids are widely used, theyrequire large, complex pumps to circulate the fluid down the drillstring and up the annulus of the well bore. As the drill is operated,the resulting cuttings from the drill bit are suspended in the drillingfluid, thereby increasing the density and further increasing the pumpingcosts. In offshore well the hydrostatic pressure put additional strainon the pumping equipment and further increase the pumping costs. Thesecost associated with use and maintenance of these pumps contributesignificantly to the costs of oil drilling operations. Further, theincreased pressures and loads on the pumps make it difficult to maintainthe pressure of the drilling fluid in the optimal range; between that ofthe pore pressure and the fracture pressure.

Several methods have been proposed to reduce the costs and overcome theproblems associated with pumping drilling fluids. Shell E&P hasintroduced the Shell Subsea Pumping System (SSPS) whereby the drillingfluid is processed, cuttings removed and discharged at the seafloor, andgas separated prior to being pumped back to the surface. Conoco hasdeveloped a dual gradient system called Subsea Mudlift in which thedrilling mud is removed from the riser with triplex pumps at theseafloor, and is then filled with seawater to reduce the riser load.Another approach called DeepVision, by Baker-Hughes and Transocean SedcoForex uses centrifugal pumps to separate the mud at the seafloor andsend it to the surface.

Some well operators have used a gas injection system to reduce thedensity of the drilling mud. In this system a gas such as nitrogen isadded to the drilling fluid, which is circulated in the conventionalmanner. However due to the compressible nature of gas, large volumes andhigh pressures are required to maintain a gas phase in the mud,increasing the complexity and cost of the system and maintaining theappropriate pressures in the well bore. Mud/gas systems have shown atendency to foam at the reduced pressures encountered as the mud/gassystem ascends the well bore or riser causing fluid handling problems.In addition, small amounts of oxygen in the injected gas have led tocorrosion problems.

To overcome the problems associated with gas injections systems, the useof hollow microspheres has been proposed. Hollow microspheres, beingrelatively incompressible, do not require the high pressures andassociated pumps necessary with gas injection and the addition ofmicrospheres will not lead to the foaming problems. However, improperlyhandled, and the size shape, density and particle size distribution canprovide a nuisance dusty environment. Further, the microspheres can bedifficult to efficiently separate and recycle from the drilling fluid,adding cost and complexity to their use.

SUMMARY OF THE INVENTION

The present invention provides a drilling fluid composition comprising adrilling fluid vehicle, which may be oil- or water-based, and acomposite microsphere component. The microspheres of the compositemicrosphere component may comprise any hollow microspheres of glass,ceramic or plastic that may be added to the drilling fluid (with othercomponents of the drilling fluid known in the art) to reduce the densitythereof. Generally the composite microsphere component is added to thedrilling composition in amounts sufficient to reduce the density atleast 15%, preferably at least 20% and most preferably at least 30%. Inone embodiment, the microsphere component is added in amounts sufficientto reduce the density of the mud to about that of the ambient seawater,or about 8 to 13 lbs/gallon (5.2 to 7.5 kg/L). In another embodiment,the microsphere component may comprise 25 to 50 volume percent of thedrilling fluid composition. Such reduction in the density of thedrilling fluid greatly reduces the pressures required to raise thedrilling fluid to the surface, and reduces the associated pumping costs.

The composite microsphere component comprises a composite ofmicrospheres and a polymeric resin. The composite microsphere componentmay be of any suitable size and shape. The composite may comprisepellets having a continuous polymeric phase having the microspheresdispersed therein, or the composite microsphere component may comprisean agglomerate of microspheres bound together by a discontinuous phaseof polymeric resin. The polymeric resin may be a thermoplastic orthermoset resin. Composites having an intermediate structure betweenpellets and agglomerates are also contemplated.

Pellets comprising a continuous phase of polymeric resin generallycomprise 20 to 75 weight % microspheres in the polymeric resin binder.The pellets may range in size from 200 to 4000 micrometers and havedensities in the range of 0.4 to 1.0 g/cm³. Agglomerates comprisesufficient polymeric resin to bind a plurality of microspheres inrandomly shaped composite particles of about 200 to 4000 micrometers andhaving densities of 0.4 to 0.7 g/cm³. Generally the agglomeratescomprise 40 to 90 weight % of the microspheres. Microsphere compositeshaving sizes in excess of about 7 mm may contribute to pumping problems.

The compressive strength required of the composite microsphere componentused in drilling applications is dictated by the depth of water at whichit will be employed: at shallow depths, the compressive strength ofmicrosphere component does not have to be high, but at very great depthsunder the sea, the hydrostatic pressure exerted on the microspherecomponent becomes enormous, and the microsphere component should havevery high resistance to compression (high compressive strengths). Hollowmicrospheres, because of their spherical form, provide resistance tocompression equally from all directions (isotropic compressivestrength), and are ideally suited for this application. Generally, themicrosphere component has a collapse strength of at least 4000 psi (27.6MPa), preferably at least 5000 psi (34.5 MPa) to provide an essentiallyincompressible density-reducing additive, in contrast to conventionalgas-injection processes.

For underwater applications, the microsphere component should havesufficient hydrolytic stability, and the resin type is chosenaccordingly. Preferred resins exhibit excellent hydrolytic stability,and in addition, offer outstanding compressive strengths. Strong resinsand strong low-density hollow glass microspheres can be advantageouslyused to meet the stringent requirements of deep water drillingapplications.

The present invention also provides a method of drilling comprising thestep of circulating a drilling fluid down a drill string and up anannulus between the drill string and bore hole, and introducing amicrosphere component to said drilling fluid in an amount sufficient toreduce the density thereof. The method may further comprise the step ofseparating the composite microsphere component from the drilling fluidcomposition and drill cuttings and the drilling fluid is returned to thesurface. To facilitate separation, the microsphere component ispreferably at least 200 micrometers in size.

The present invention also provides a method of reducing the density ofthe drilling fluid composition by adding a composite microspherecomponent to the drilling fluid composition in amounts sufficient toreduce the density at least 15%, preferably at least 20% and mostpreferably at least 30%. In one embodiment, the microsphere component isadded in amounts sufficient to reduce the density of the mud to aboutthat of the ambient seawater, or about 8 to 13 lbs/gallon (5.2 to 7.5kg/L).

The invention provides a reduced density drilling fluid composition andmethod of drilling that advantageously reduces the density of the fluidand reduces costs associated with pumping. The invention usesconventional drilling and pumping equipment, requires no sea floor basedpumps and may be easily pressure-controlled to maintain the pressure ofthe fluid to that of the ambient water pressure. More specifically, thepressure of the fluid may be maintained between fracture pressure andthe pore pressure of the well to avoid fracturing the well formationand/or reduce the infiltration of water (or other fluids) from the poresof the well formation.

Advantageously the use of a composite microsphere component overcomesproblems inherent in gas-injection processes by providing an essentiallyincompressible additive that may be used to reduce the density of adrilling fluid. The composites also allow one to specifically tailor thedensity, strength and size of the additive to the specific well drillingrequirements and facilitates separation due to the larger size (ascompared to unitary microspheres).

DETAILED DESCRIPTION

The microspheres used in the composite microsphere component may be anytype of hollow spheres that are known to the art. The microspheres arepreferably made of glass, but may be made be polymeric, ceramic or othermaterials known to the art, provided the microsphere component hassufficient physical properties to withstand the severe conditionsencountered in well drilling, including collapse strength, hydrolyticstability, size, density and compatibility with polymeric resins.

Useful microspheres (of the composite) are hollow, generally round butneed not be perfectly spherical; they may be cratered or ellipsoidal,for example. Such irregular, though generally round or spherical, hollowproducts are regarded as “microspheres” herein.

The microspheres of the composite are generally from about 5 to 1000micrometers in diameter, and are preferably 50 and 500 micrometers indiameter. Microspheres comprising different sizes or a range of sizesmay be used. Where the microsphere component comprises a composite ofmicrospheres and a resin, for example in the form of an agglomerate or apellet, the size of the unitary microspheres is less significant sincethe composite particle may be sized appropriately to facilitateseparation and recovery.

As the microspheres are subjected to high pressures in a well, themicrospheres should have a collapse strength in excess of theanticipated pressures. Generally the microsphere component should have aburst strength in excess of 4000 psi (27.6 MPa), preferably in excess5000 psi (34.5 MPa) as measured by ASTM D3102-78 with 10% collapse andpercent of total volume instead of void volume as stated in the test.

The density of the microspheres may vary from about 0.1 to 0.9 g/cm³,and is preferably in the range of 0.2 to 0.7 g/cm³. When a microspherecomposite is used, the agglomerate having a discontinuous phase ofpolymeric resin may have densities in the range of 0.4 to 0.7 g/cm³, andcomposite pellets having a continuous phase of polymeric resin, may havedensities in the range of 0.4 to 1.0 g/cm³.

Glass microspheres have been known for many years, as is shown byEuropean Patent 0 091,555, and U.S. Pat. Nos. 2,978,340, 3,030,215,3,129,086 3,230,064, and U.S. Pat. No. 2,978,340, all of which teach aprocess of manufacture involving simultaneous fusion of theglass-forming components and expansion of the fused mass. U.S. Pat. No.3,365315 (Beck), U.S. Pat. No. 4,279,632 (Howell), U.S. Pat. No.4,391,646 (Howell) and U.S. Pat. No. 4,767,726 (Marshall) teach analternate process involving heating a glass composition containing aninorganic gas forming agent, and heating the glass to a temperaturesufficient to liberate the gas and at which the glass has viscosity ofless than about 104 poise.

Useful glass microspheres have a density of at least 0.1 gram per cubiccentimeter, which is equivalent to a ratio of wall thickness to bubblediameter of about 0.029. Density is determined (according to ASTMD-2840-69) by weighing a sample of microspheres and determining thevolume of the sample with an air comparison pycnometer (such as aAccuPyc 1330 Pycnometer or a Beckman Model 930). Higher densities canproduce higher strengths, and densities of 0.5 or 0.6 g/cm³ or more arepreferred for some uses. The microspheres generally have an averagediameter between about 5 and 1000 micrometers, and preferably betweenabout 50 and 500 micrometers. Size can be controlled by the amount ofsulfur-oxygen compounds in the particles, the length of time that theparticles are heated, and by other means known in the art. Themicrospheres may be prepared on apparatus well known in the microspheresforming art, e.g., apparatus similar to that described in U.S. Pat. No.3,230,064 or 3,129,086.

One method of preparing glass microspheres is taught in U.S. Pat. No.3,030,215, which describes the inclusion of a blowing agent in anunfused raw batch of glass-forming oxides. Subsequent heating of themixture simultaneously fuses the oxides to form glass and triggers theblowing agent to cause expansion. U.S. Pat. No. 3,365,315 describes animproved method of forming glass microspheres in which pre-formedamorphous glass particles are subsequently reheated and converted intoglass microspheres. U.S. Pat. No. 4,391,646 discloses that incorporating1-30 weight percent of B₂O₃, or boron trioxide, in glasses used to formmicrospheres, as in U.S. Pat. No. 3,365,315, improves strength, fluidproperties, and moisture stability. A small amount of sodium borateremains on the surface of these microspheres, causing no problem in mostapplications. Removal of the sodium borate by washing is possible, butat a significant added expense; even where washing is carried out,however, additional sodium borate leaches out over a period of time.

Hollow glass microspheres are preferably prepared as described in U.S.Pat. No. 4,767,726 (Marshall), incorporated herein by reference, due tothe greater hydrolytic stability. These microspheres are made from aborosilicate glass and have a chemical composition consistingessentially of SiO₂, CaO, Na₂O, B₂O₃, and SO₃ blowing agent. Acharacterizing feature of the microspheres resides in the alkaline metalearth oxide:alkali metal oxide (RO:R₂O) ratio, which substantiallyexceeds 1:1 and lies above the ratio present in any previously utilizedsimple borosilicate glass compositions. As the RO:R₂O ratio increasesabove 1:1, simple borosilicate compositions become increasinglyunstable, devitrifying during traditional working and cooling cycles, sothat “glass” compositions are not possible unless stabilizing agentssuch as Al₂O₃ are included in the composition. Such unstablecompositions have been found to be highly desirable for making glassmicrospheres, rapid cooling of the molten gases by water quenching, toform frit, preventing devitrification. During subsequent bubble forming,as taught in aforementioned U.S. Pat. Nos. 3,365,315 and 4,391,646, themicrospheres cool so rapidly that devitrification is prevented, despitethe fact that the RO:R₂O ratio increases even further because of loss ofthe relatively more volatile alkali metal oxide compound during forming.

These microspheres have a density ranging from 0.08 or less to about 0.8g/cc, the less dense products being more economical per unit volume.Glass microspheres having a higher density are, however, particularlyuseful in the present invention where an inexpensive and comparativelylightweight microspheres having high resistance to crushing is desired.These microspheres, in which the chemical composition, expressed inweight percent, consists essentially of at least 70% SiO₂, 8-15% RO,3-8% R₂ O, 2-6% B₂O₃, and 0.125-1.50% SO₃, the foregoing componentsconstituting at least about 90% (preferably 94% and still morepreferably 97%) of the glass, the RO:R₂O weight ratio being in the rangeof 1.2-3.5.

Preparation of hollow, ceramic microspheres by spray drying is taught inU.S. Pat. No. 4,421,562. U.S. Pat. No. 4,637,990 describes hollow,ceramic, porous microspheres prepared by a blowing technique. Theresultant ceramic microspheres have diameters of 2000 to 4000micrometers.

U.S. Pat. No. 4,279,632 discloses a method and apparatus for producingconcentric hollow spheres by a vibration technique on extruded materialsto break up the material into individual, spherical bodies. This methodis useful with low melting point material such as glass or metal whichis fluid at elevated operating temperatures.

Hollow ceramic balls prepared by a combination of coating, sintering,and reduction are disclosed in U.S. Pat. No. 4,039,480; however, theprocess is complex, and the balls so obtained are large (e.g., 5 by 7mesh size which is 2.79 to 3.96 millimeters).

Ceramic metal oxide microspheres prepared by impregnating hollow,organic resin microspheres with a metal compound and firing to removeadjuvants is disclosed in U.S. Pat. No. 3,792,136. The resultant hollowmicrospheres generally have large diameters of 50 micrometers to 10millimeters (mm) and in one example, when the average diameter was 3 mm,the wall thickness is disclosed to be 17 micrometers.

U.S. Pat. No. 2,978,340 describes inorganic microspheres prepared from afusion (melt or vitreous) process using a gassing agent. The product isnot uniform in size, and the microspheres are not all hollow.

Hollow ceramic spheres of low density may be prepared by the processtaught in U.S. Pat. Nos. 4,111,713, and 4,744,831, which comprises

-   -   (A) tumbling together and thoroughly mixing (1) solidifiable        liquid globules comprising a thermally fugitive organic binder        material and a source of void-forming agent adapted to evolve as        a gas and convert the liquid globules to a hollow condition        and (2) a mass of minute discrete free-flowing inorganic        heat-sinterable parting agent particles selected from metals,        metalloids, metal oxides and metal salts that are wetted by, and        at least partially absorbed into, the liquid globules during the        tumbling action; sufficient parting agent particles being        present so that any portion of liquid globules uncovered by        parting agent particles tumble against discrete unabsorbed        parting agent particles;    -   (B) providing conditions during the tumbling action, and        tumbling for a sufficient time, for the void-forming agent to        evolve as a gas and form a central interior space within the        liquid globules and for the thus-hollowed liquid globules to        solidify;    -   (C) collecting the converted globules after they have solidified        to a shape-retaining condition; and    -   (D) firing the hollow spheres to first burn out the organic        binder, and to then sinter the parting agent particles to form        hollow shape-retaining spheres.

Another useful ceramic microsphere is taught in U.S. Pat. No. 5,077,241(Moh, et al.) which comprises microspheres consisting essentially of atleast one of a non-oxide component (or phase) and an oxide component (orphase), each microsphere having a ceramic wall and a single centralcavity, the microspheres having diameters in the range of 1 to 300micrometers and wall thicknesses of less than 10 percent of the diameterof the microspheres. Such ceramic microspheres may be prepared by

-   (1) providing a mixture containing a ceramic sol precursor and a    volatile liquid, the volatile liquid being referred to herein as    bloating agent,-   (2) adding the above mixture, preferably as droplets, at a suitable    rate and manner to a provided bubble promoting medium maintained at    a suitable temperature to allow formation of green hollow    microspheres; preferably the bubble promoting medium is a liquid    such as an aliphatic alcohol, e.g. oleyl alcohol, or a long chain    carboxylic acid ester such as peanut oil, or mixtures thereof, or    mixtures of oleyl alcohol with other vegetable oils or vegetable oil    derivatives,-   (3) isolating the green microspheres, preferably by filtration, and-   (4) firing the green microspheres, optionally mixed with an    agglomeration preventative agent to provide a source of carbon, in    air for oxide containing ceramic microspheres or in an inert or    reducing atmosphere for non-oxide containing microspheres, and at a    range of temperature sufficient to convert the green microspheres    into an oxide or non-oxide containing ceramic.

Useful polymeric microspheres may be prepared by the general method ofpolymerization of polymeric particles having a minor amount of avolatile blowing agent dissolved within the particles which expands onheating. U.S. Pat. No. 3,615,972 (Morehouse et al.) describesthermoplastic microspheres that encapsulate a liquid blowing agent. Themicrospheres are prepared by suspension polymerization of droplets of amixture of monomer(s) and a blowing agent. U.S. Pat. No. 3,472,798(Pitchforth et al.) described the preparation of polymethylmethacrylateprepared by suspension polymerization. U.S. Pat. No. 3,740,359 (Garner)and U.S. Pat. No. 4,075,138 (Garner) describes vinylidine chloridecopolymer microspheres prepared from an oil phase of the monomers and aliquid blowing agent, dispersing the oil phase in an aqueous phasecontaining a dispersion stabilizer, polymerizing the monomers, thenheating to volatilize the blowing agent. U.S. Pat. No. 3,945,956(Garner) described expandable styrene-acrylonitrile microspheresprepared by polymerizing a mixture of styrene and acrylonitrile with avolatile liquid blowing agent.

The microsphere component may comprise a composite comprising aplurality of hollow glass, ceramic or plastic microspheres bondedtogether with a polymeric binder. The binder may be continuous (as in aparticle or pellet), or discontinuous (as in an agglomerate) or anintermediate structure. As such, the amount of microspheres in thecomposite can vary widely; from about 20 to 75, preferably 20 to 60weight % to form a pellet composite and 40 to 95, preferably 40 to 90weight % to form an agglomerate. The microsphere composites may be ofany suitable size or shape are typically at least 200 micrometers insize, and preferably 4000 micrometers or less to facilitate subsequentseparation from the drilling fluid. The composites may be any desiredshape including random or regular shapes.

Thermoplastic polymers may be used as a binder in the compositemicrosphere. Thermoplastic polymers which may be used in the presentinvention include but are not limited to melt-processible polyolefinsand copolymers and blends thereof, styrene copolymers and terpolymers(such as Kraton™), ionomers (such as Surlyn™), ethyl vinyl acetate (suchas Elvax™), polyvinylbutyrate, polyvinyl chloride, metallocenepolyolefins (such as Affinity™ and Engage™), poly(alpha olefins) (suchas Vestoplast™ and Rexflex™), ethylene-propylene-diene terpolymers,fluorocarbon elastomers (such as THV™ from 3M Dyneon), otherfluorine-containing polymers, polyester polymers and copolymers (such asHytrel™), polyamide polymers and copolymers, polyurethanes (such asEstane™ and Morthane™), polycarbonates, polyketones, and polyureas. Thethermoplastic polymers include blends of homo- and copolymers, as wellas blends of two or more homo- or copolymers. As used herein“melt-processible” refers to thermoplastic polymers having a melt indexof from 3 to 30 g/10 min.

Useful polyamide polymers include, but are not limited to, syntheticlinear polyamides, e.g., nylon-6 and nylon-66, nylon-11, or nylon-12. Itshould be noted that the selection of a particular polyamide materialmight be based upon the physical requirements of the particularapplication for the resulting reinforced composite article. For example,nylon-6 and nylon-66 offer higher heat resistant properties thannylon-11 or nylon-12, whereas nylon-11 and nylon-12 offer betterchemical resistant properties. In addition to those polyamide materials,other nylon materials such as nylon-612, nylon-69, nylon-4, nylon-42,nylon-46, nylon-7, and nylon-8 may also be used. Ring containingpolyamides, e.g., nylon-6T and nylon-61 may also be used. Polyethercontaining polyamides, such as PEBAX polyamides (Atochem North America,Philadelphia, Pa.), may also be used.

Polyurethane polymers which can be used include aliphatic,cycloaliphatic, aromatic, and polycyclic polyurethanes. Thesepolyurethanes are typically produced by reaction of a polyfunctionalisocyanate with a polyol according to well-known reaction mechanisms.Commercially available urethane polymers useful in the present inventioninclude: PN-04 or 3429 from Morton International, Inc., Seabrook, N.H.,and X4107 from B.F.Goodrich Company, Cleveland, Ohio.

Also useful are polyacrylates and polymethacrylates which include, forexample, polymers of acrylic acid, methyl acrylate, ethyl acrylate,acrylamide, methylacrylic acid, methyl methacrylate, n-butyl acrylate,and ethyl acrylate, to name a few.

Other useful substantially extrudable hydrocarbon polymers includepolyesters, polycarbonates, polyketones, and polyureas. These materialsare generally commercially available, for example: SELAR® polyester(DuPont, Wilmington, Del.); LEXAN® polycarbonate (General Electric,Pittsfield, Mass.); KADEL® polyketone (Amoco, Chicago, Ill.); andSPECTRIM® polyurea (Dow Chemical, Midland, Mich.).

Useful fluorine-containing polymers include crystalline or partiallycrystalline polymers such as copolymers of tetrafluoroethylene with oneor more other monomers such as perfluoro(methyl vinyl)ether,hexafluoropropylene, perfluoro(propyl vinyl)ether; copolymers oftetrafluoroethylene with ethylenically unsaturated hydrocarbon monomerssuch as ethylene, or propylene.

Still other fluorine-containing polymers useful in the invention includethose based on vinylidene fluoride such as polyvinylidene fluoride;copolymers of vinylidene fluoride with one or more other monomers suchas hexafluoropropylene, tetrafluoroethylene, ethylene, propylene, etc.Still other useful fluorine-containing extrudable polymers will be knownto those skilled in the art as a result of this disclosure.

Representative examples of polyolefins useful in this invention arepolyethylene, polypropylene, polybutylene, poly(1-butene),poly(3-methylbutene), poly(4-methylpentene) and copolymers of ethylenewith propylene, 1-butene, 1-hexene, 1-octene, 1-decene,4-methyl-1-pentene and 1-octadecene, and blends thereof. Usefulcommercially available polyolefins include MOPLEN and ADFLEX KS359polypropylene available from Basell, Bloomington, Del., SRC 7644polypropylene available from Exxon/Mobil, Edison, N.J.

Representative blends of polyolefins useful in this invention are blendscontaining polyethylene and polypropylene, low-density polyethylene andhigh-density polyethylene, and polyethylene and olefin copolymerscontaining the copolymerizable monomers, some of which are describedabove, e.g., ethylene and acrylic acid copolymers; ethyl and methylacrylate copolymers; ethylene and ethyl acrylate copolymers; ethyleneand vinyl acetate copolymers-, ethylene, acrylic acid, and ethylacrylate copolymers, and ethylene, acrylic acid, and vinyl acetatecopolymers.

The useful thermoplastic polyolefins may also comprise functionalizedpolyolefins, i.e., polyolefins that have additional chemicalfunctionality, obtained through either copolymerization of olefinmonomer with a functional monomer or graft copolymerization subsequentto olefin polymerization. Typically, such functionalized groups includeO, N, S, P, or halogen heteroatoms. Such reactive functionalized groupsinclude carboxylic acid, hydroxyl, amide, nitrile, carboxylic acidanhydride, or halogen groups. Many functionalized polyolefins areavailable commercially. For example, copolymerized materials includeethylene-vinyl acetate copolymers, such as the Elvax series,commercially available from DuPont Chemicals, Wilmington, Del., theElvamide series of ethylene-polyamide copolymers, also available fromDuPont, and Abcite 1060WH, a polyethylene-based copolymer comprisingapproximately 10% by weight of carboxylic acid functional groups,commercially available from Union Carbide Corp., Danbury, Conn. Examplesof graft-copolymerized functionalized polyolefins include maleicanhydride-grafted polypropylene, such as the Epolene series commerciallyavailable from Eastman Chemical Co., Kingsport, Tenn. and Questron,commercially available from Himont U.S.A., Inc., Wilmington, Del.

Thermoplastic microsphere composites can be prepared using anyconventional technique for preparing particle-filled thermoplasticarticles. The thermoplastic polymer can be heated above its meltingpoint and the microspheres can then be mixed in. The resulting mixturemay then be extruded or formed into continuous strands and the strandsare cooled to solidify the moldable polymer for pelletizing on suitableequipment as is known in the art. Alternatively, a molten mixture ofthermoplastic polymer and microsphere may be discharged using apelletizing spray apparatus as is known in the art.

In a preferred method of making a microsphere composite, themicrospheres, preferably glass microspheres are metered into a moltenstream of thermoplastic polymer under low shear conditions to form amixture, and the mixture is then formed into the desired size and shape.This process may comprise a two-stage extrusion process whereby athermoplastic polymer is melted in the first stage of an extruder andconveyed to a second stage, where the microspheres are added to themolten stream. The microspheres and the thermoplastic resin are mixed inthe second stage, the mixture degassed and extruded in the desired form.

Thermoset polymers may be used as the binder for the compositemicrosphere. As used herein, thermoset refers to a polymer thatsolidifies or sets irreversibly when cured. Curable binder precursor canbe cured by radiation energy or thermal energy. Thermosettablecompositions may include components that have a radiation or heatcrosslinkable functionality such that the composition is curable uponexposure to radiant curing energy in order to cure and solidify, i.e.polymerize and/or crosslink, the composition. Representative examples ofradiant curing energy include, electromagnetic energy (e.g., infraredenergy, microwave energy, visible light, ultraviolet light, and thelike), accelerated particles (e.g., electron beam energy), and/or energyfrom electrical discharges (e.g., coronas, plasmas, glow discharge, orsilent discharge).

Radiation crosslinkable functionality refers to functional groupsdirectly or indirectly pendant from a monomer, oligomer, or polymerbackbone that participate in crosslinking and/or polymerizationreactions upon exposure to a suitable source of radiant curing energy.Such functionality generally includes not only groups that crosslink viaa cationic mechanism upon radiation exposure but also groups thatcrosslink via a free radical mechanism. Representative examples ofradiation crosslinkable groups suitable in the practice of the presentinvention include epoxy groups, (meth)acrylate groups, olefiniccarbon-carbon double bonds, allylether groups, styrene groups,(meth)acrylamide groups, combinations of these, and the like.

Typically, radiation curable binder precursor material comprises atleast one of epoxy resin, acrylated urethane resin, acrylated epoxyresin, ethylenically unsaturated resin, aminoplast resin having at leastone pendant unsaturated carbonyl group, isocyanurate derivatives havingat least one pendant acrylate group, isocyanate derivatives having atleast one pendant acrylate group, or combinations thereof. Othersuitable thermoset polymers include those derived from phenylic resins,vinyl ester resins, vinyl ether resins, urethane resins, cashew nutshell resins, napthalinic phenylic resins, epoxy modified phenylicresins, silicone (hydrosilane and hydrolyzable silane) resins, polyimideresins, urea formaldehyde resins, methylene dianiline resins,methylpyrrolidinone resins, acrylate and methacrylate resins, isocyanateresins, unsaturated polyester resins, and mixtures thereof.

A polymer precursor or precursors may be provided to form the desiredthermoset polymer. The polymer precursor or thermoset resin may comprisemonomers, or may comprise a partially polymerized, low molecular weightpolymer, such as an oligomer, if desired. Solvent or curative agent,such as a catalyst, may also be provided where required. In one method,the microsphere composite may be prepared by mixing the microsphereswith a polymer precursor or resin and subsequently curing the polymerprecursor or resin. A solvent, if any, may be removed by evaporation.The evaporation and polymerization may take place until thepolymerization is substantially complete.

Epoxy (epoxide) monomers and prepolymers are commonly used in makingthermoset epoxy materials, and are well known in the art. Thermosettableepoxy compounds can be cured or polymerized by cationic polymerization.The epoxy-containing monomer can also contain other epoxy compounds orblends of epoxy containing monomers with thermoplastic materials. Theepoxy-containing monomer may be blended with specific materials toenhance the end use or application of the cured, or partially cured,composition.

Useful epoxy-containing materials include epoxy resins having at leastone oxirane ring polymerizable by a ring opening reaction. Suchmaterials, broadly called epoxides, include both monomeric and polymericepoxides, and can be aliphatic, cycloaliphatic, or aromatic. Thesematerials generally have, on the average, at least two epoxy groups permolecule, and preferably have more than two epoxy groups per molecule.The average number of epoxy groups per molecule is defined herein as thenumber of epoxy groups in the epoxy-containing material divided by thetotal number of epoxy molecules present. Polymeric epoxides includelinear polymers having terminal epoxy groups (e.g., a diglycidyl etherof a polyoxyalkylene glycol), polymers having skeletal oxirane units(e.g., polybutadiene polyepoxide), and polymers having pendent epoxygroups (e.g., a glycidyl methacrylate polymer or copolymer). Themolecular weight of the epoxy-containing material may vary from 58 toabout 100,000 or more. Mixtures of various epoxy-containing materialscan also be used.

Examples of some epoxy resins useful in this invention include2,2-bis[4-(2,3-epoxypropyloxy)phenyl]propane (diglycidyl ether ofbisphenyl A) and materials under the trade designation “EPON 828”, “EPON1004” and “EPON 1001F”, commercially available from Shell Chemical Co.,Houston, Tex., “DER-331”, “DER-332” and “DER-334”, commerciallyavailable from Dow Chemical Co., Freeport, Tex., Other suitable epoxyresins include glycidyl ethers of phenyl formaldehyde novolac (e.g.,“DEN-431” and “DEN-428”, commercially available from Dow Chemical Co.)and BLOX 220 thermoplastic epoxy resin available from Dow, Midland,Mich. The epoxy resins used in the invention can polymerize via acationic mechanism with the addition of appropriate photoinitiator(s).These resins are further described in U.S. Pat. Nos. 4,318,766 and4,751,138, which are incorporated by reference.

Exemplary acrylated urethane resin includes a diacrylate ester of ahydroxy terminated isocyanate extended polyester or polyether. Examplesof commercially available acrylated urethane resin include “UVITHANE782” and “UVITHANE 783,” both available from Morton Thiokol Chemical,Moss Point, Miss., and “CMD 6600”, “CMD 8400”, and “CMD 8805”, allavailable from Radcure Specialties, Pampa, Tex.

Exemplary acrylated epoxy resin includes a diacrylate ester of epoxyresin, such as the diacrylate ester of an epoxy resin such as bisphenyl.Examples of commercially available acrylated epoxy resin include “CMD3500”, “CMD 3600”, and “CMD 3700”, available from Radcure Specialties.

Exemplary ethylenically unsaturated resin includes both monomeric andpolymeric compounds that contain atoms of carbon, hydrogen and oxygen,and optionally, nitrogen or the halogens. Oxygen atoms, nitrogen atoms,or both, are generally present in ether, ester, urethane, amide, andurea groups. Ethylenically unsaturated resin typically has a molecularweight of less than about 4,000 and is in one embodiment an esterresulting from the reaction of compounds containing aliphaticmonohydroxy groups or aliphatic polyhydroxy groups and unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid,crotonic acid, isocrotonic acid, maleic acid, and the like.

Representative examples of other useful acrylates include methylmethacrylate, ethyl methacrylate, ethylene glycol diacrylate, ethyleneglycol methacrylate, hexanediol diacrylate, triethylene glycoldiacrylate, trimethylolpropane triacrylate, glycerol triacrylate,pentaerythritol triacrylate, pentaerythritol methacrylate, andpentaerythritol tetraacrylate. Other useful ethylenically unsaturatedresins include monoallyl, polyallyl, and polymethylallyl esters andamides of carboxylic acids, such as diallyl phthalate, diallyl adipate,and N,N-diallyladipamide. Still, other useful ethylenically unsaturatedresins include styrene, divinyl benzene, and vinyl toluene. Other usefulnitrogen-containing, ethylenically unsaturated resins includetris(2-acryloyl-oxyethyl)isocyanurate,1,3,5-tri(2-methyacryloxyethyl)-s-triazine, acrylamide,methylacrylamide, N-methylacrylamide, N,N-dimethylacrylamide,N-vinylpyrrolidone, and N-vinylpiperidone.

Some useful aminoplast resins can be monomeric or oligomeric. Typically,the aminoplast resins have at least one pendant α,β-unsaturated carbonylgroup per molecule. These α,β-unsaturated carbonyl groups can beacrylate, methacrylate, or acrylamide groups. Examples of such resinsinclude N-hydroxymethyl-acrylamide, N,N′-oxydimethylenebisacrylamide,ortho and para acrylamidomethylated phenyl, acrylamidomethylatedphenylic novolac, and combinations thereof. These materials are furtherdescribed in U.S. Pat. Nos. 4,903,440 and 5,236,472, which areincorporated by reference.

Useful isocyanurate derivatives having at least one pendant acrylategroup and isocyanate derivatives having at least one pendant acrylategroup are further described in U.S. Pat. No. 4,652,274, which isincorporated by reference. One such isocyanurate material is atriacrylate of tris(2-hydroxyethyl)isocyanurate.

Examples of vinyl ethers suitable for this invention include vinyl etherfunctionalized urethane oligomers, commercially available from AlliedSignal, Morristown, N.J., under the trade designations “VE 4010”, “VE4015”, “VE 2010”, “VE 2020”, and “VE 4020”.

Phenylic resins are low cost, heat resistant, and have excellentphysical properties. Acid cure resole phenylic resins are disclosed inU.S. Pat. No. 4,587,291. Phenyl resins used in some embodiments of theinvention can have a content of monomeric phenyls of less than 5%. Theresins can also be modified additionally with up to 30% of urea,melamine, or furfuryl alcohol, according to known methods.

Phenyl resoles are alkaline condensed, reaction products of phenyls andaldehydes, wherein either mono- or polynuclear phenyls may be used. Infurther detail, mononuclear phenyls, and both mono- and polyfunctionalphenyls, such as phenyl itself, and the alkyl substituted homologs, suchas o-, m-, p-cresol or xylenols, are suitable. Also suitable arehalogen-substituted phenyls, such as chloro- or bromophenyl andpolyfunctional phenyls, such as resorcinol or pyrocatechol. The term“polynuclear phenyls” refers, for example, to naphthols, i.e., compoundswith fused rings. Polynuclear phenyls may also be linked by aliphaticbridges or by heteroatoms, such as oxygen. Polyfunctional, polynuclearphenyls may also provide suitable thermosetting phenyl resoles.

The aldehyde component used to form the phenyl resoles can beformaldehyde, acetaldehyde, propionaldehyde, or butyraldehyde, orproducts that release aldehyde under condensation conditions, such as,for example, formaldehyde bisulfite, urotropin, trihydroxymethylene,paraformaldehyde, or paraldehyde. The stoichiometric quantities ofphenyl and aldehyde components can be in the ratio of 1:1.1 to 1:3.0.The resins can be used in the form of aqueous solutions with a contentof non-volatile substances of 60 to 85%.

Oxetane ring monomers may also be used to form the matrix phasethermoset polymers. Oxetane (oxacyclobutane) rings behave somewhat likeepoxy (oxirane) rings in that catalysts and/or co-curatives, sometimesreferred to as crosslinking agents, can be used to open the ring andlink two or more chains together to form a crosslinked polymer. Forexample, polycarboxylic acid anhydrides and other polyfunctionalcompounds such as polyamines, polycarboxylic acids, polymercaptans,polyacid halides, or the like are capable of linking two or more oxetanesites just as epoxy sites are linked by epoxide cocuratives. The resultis an increased amount of three-dimensional structure in the crosslinkedor cured polymer, and hence an increased amount of rigidity of thepolymer structure.

The mixture of microspheres and curable binder precursor material may becured by an initiator selected from the group consisting ofphotoinitiator, thermal initiator, and combinations thereof. As usedherein, a thermal initiator may be used when thermal energy is used inthe at least partially curing step, and photoinitiators may be used whenultraviolet and/or visible light is used in the at least partiallycuring step. The requirement of an initiator may depend on the type ofthe curable binder precursor used and/or the type of energy used in theat least partially curing step (e.g., electron beam or ultravioletlight). For example, phenylic-based curable binder precursors typicallydo not require the addition of an initiator when at least thermallycured. However, acrylate-based curable binder precursors typically dorequire the addition of an initiator when at least thermally cured. Asanother example, initiators typically are not required when electronbeam energy is used during the at least partially curing step. However,if ultraviolet or visible light is utilized, a photoinitiator istypically included in the composition.

Upon being exposed to thermal energy, a thermal initiator generates afree radical source. The free radical source then initiates thepolymerization of the curable binder precursor. Exemplary thermalinitiators include organic peroxides (e.g. benzoil peroxide), azocompounds, quinones, nitroso compounds, acyl halides, hydrazones,mercapto compounds, pyrylium compounds, imidazoles, chlorotriazines,benzoin, benzoin alkyl ethers, diketones, phenones, and mixturesthereof. Azo compounds suitable as thermal initiators in the presentinvention may be obtained under the trade designations “VAZO 52,” “VAZO64,” and “VAZO 67” from E.I. duPont deNemours and Co., Wilmington, Del.

Upon being exposed to ultraviolet or visible light, the photoinitiatorgenerates a free radical source or a cationic source. This free radicalor cationic source then initiates the polymerization of the curablebinder precursor.

Exemplary photoinitiators that generate a free radical source whenexposed to ultraviolet light include, but are not limited to, thoseselected from the group consisting of organic peroxides (e.g., benzoylperoxide), azo compounds, quinones, benzophenones, nitroso compounds,acyl halides, hydrozones, mercapto compounds, pyrylium compounds,triacrylimidazoles, bisimidazoles, chloroalkytriazines, benzoin ethers,benzil ketals, thioxanthones, and acetophenone derivatives, and mixturesthereof. Examples of photoinitiators that generate a free radical sourcewhen exposed to visible radiation are further described, for example, inU.S. Pat. No. 4,735,632 (Oxman et al.), the disclosure of which isincorporated herein by reference.

Cationic photoinitiators generate an acid source to initiate thepolymerization of an epoxy resin or a urethane. Exemplary cationicphotoinitiators include a salt having an onium cation and ahalogen-containing complex anion of a metal or metalloid. Other usefulcationic photoinitiators include a salt having an organometallic complexcation and a halogen-containing complex anion of a metal or metalloid.These photoinitiators are further described in U.S. Pat. No. 4,751,138(Tumey et al.), the disclosure of which is incorporated herein byreference. Another example is an organometallic salt and an onium saltdescribed in U.S. Pat. No. 4,985,340 (Palazotto et al.); the disclosureof which is incorporated herein by reference. Still other cationicphotoinitiators include an ionic salt of an organometallic complex inwhich the metal is selected from the elements of Periodic Groups IVB,VB, VIIB, VIIB, and VIIIB. These photoinitiators are further describedin U.S. Pat. No. 5,089,536 (Palazotto), the disclosure of which isincorporated herein by reference.

Ultraviolet-activated photoinitiators suitable for the present inventionmay be obtained under the trade designations “IRGACURE 651”, “IRGACURE184”, “IRGACURE 369” and “IRGACURE 819” from Ciba Geigy Company,Winterville, Miss., “Lucirin TPO-L”, from BASF Corp., Livingston, N.J.,and “DAROCUR 1173” from Merck & Co., Rahway, N.J. In one embodiment, thetotal amount of initiator (either photoinitiator, thermal initiator, orcombinations thereof) may be in the range from 0.1 to 10 percent byweight of the curable binder precursor; in another embodiment, fromabout 1 to about 5 percent by weight of the curable binder precursor. Ifboth photoinitiator and thermal initiator are used, the ratio ofphotoinitiator to thermal initiator is between about 3.5:1 to about 1:1.

When using a thermoset resin, the microsphere composite may be preparedby forming precursor particles comprising the thermoset resin binder andmicrospheres and curing the particles. In a preferred embodiment, thefirst step involves forcing the binder and microspheres through aperforated substrate to form agglomerate precursor particles. Next, theagglomerate precursor particles are separated from the perforatedsubstrate and irradiated with radiation energy to provide agglomerateparticles. In a preferred embodiment, the method of forcing, separatingand irradiating steps are spatially oriented in a vertical andconsecutive manner, and are performed in a sequential and continuousmanner. Preferably, the agglomerate particles are solidified andhandleable after the irradiation step and before being collected.Reference may be made to U.S. Pat. No. 6,620,214 and incorporated hereinby reference.

Methods of forcing the binder precursor and solid particulates through aperforated substrate comprise extrusion, milling, calendering orcombinations thereof. In a preferred embodiment, the method of forcingis provided by a size reduction machine, manufactured by QuadroEngineering Incorporated.

In one embodiment, the agglomerate precursor particles are irradiated bybeing passing through a first curing zone that contains a radiationsource. Preferred sources of radiation comprise electron beam,ultraviolet light, visible light, laser light or combinations thereof.In another embodiment, the agglomerate particles are passed through asecond curing zone to be further cured. Preferred energy sources in thesecond curing zone comprise thermal, electron beam, ultraviolet light,visible light, laser light, microwave or combinations thereof.

In a preferred embodiment, the composite particles are filamentaryshaped and have a length ranging from about 100 to about 5000micrometers (prior to sizing). Most preferably, the filamentary shapedcomposite particles range in length from about 200 to about 1000micrometers. In one embodiment, the agglomerate particles may then bereduced in size after either the first irradiation step or after beingpassed through the second curing zone. The preferred method of sizereducing is with a size reduction machine manufactured by QuadroEngineering Incorporated. In one embodiment, the cross-sectional shapesof the agglomerate particles comprise circles, polygons or combinationsthereof. Preferably, the cross-sectional shape is constant. Furtherdetails regarding the process may be found in U.S. Pat. No. 6,620,214,incorporated herein by reference.

Agglomerates that contain a discontinuous binder can be made accordingto the following procedure. The microspheres and the binder resin areintroduced into a mixing vessel. The resulting mixture is stirred untilit is homogeneous. It is preferred that there be sufficient liquid inthe mixture that the resulting mixture is neither excessively stiff norexcessively runny. Most resins contain sufficient liquid to permitadequate mixing. After the mixing step is complete, the mixture iscaused to solidify, preferably by means of heat or radiation energy.Solidification results from either the removal of liquid from themixture or the polymerization of the resinous adhesive. After themixture is solidified, it is crushed to form agglomerates, which arethen graded to the desired size. Devices suitable for this step includeconventional jaw crushers and roll crushers.

If the binder of the agglomerate is a thermoplastic, it is preferredthat the agglomerate be made according to the following procedure. Thethermoplastic is heated to just above its melting temperature. Then theheated thermoplastic and the microspheres are introduced into a heatedscrew type extruder, and mixed until it is homogeneous. Next, themixture is run through the die of the extruder, and the resultingextrudate is cooled and crushed to form agglomerates, which are thengraded to the desired size.

The crushing and grading procedures described above frequently provideagglomerates of an undesirable size. The improperly sized agglomeratescan either be recycled, e.g., by being added to a new dispersion, ordiscarded.

The present invention provides a drilling fluid composition comprising adrilling fluid, which may be oil- or water-based, and a compositemicrosphere component. The microsphere component comprises a compositeof microspheres in a polymeric resin. The microsphere of the compositemicrosphere component may comprise any hollow microspheres of glass,ceramic or plastic that may be added to the drilling fluid (with othercomponents of the drilling fluid known in the art) to reduce the densitythereof. The composite microsphere component may be of any suitable sizeand shape. The polymeric resin may comprise a continuous phase havingthe microspheres dispersed therein, or the composite microspherecomponent may comprise an agglomerate of microspheres bound together bya discontinuous phase of polymeric resin. The polymeric resin may be athermoplastic or thermoset resin.

The composite microsphere component is added to the drilling fluidcomposition in amounts sufficient to reduce the density of the drillingfluid at least 15%, preferably at least 20% and most preferably at leastabout 30%. Normally the drilling fluid has a density in the range ofabout 15 lbs/gal (˜8.7 kg/L). One useful drilling fluid comprises amicrosphere component in an amount sufficient to reduce the density ofthe drilling to approximately that of seawater, or about 8 to 12 lbs/gal(˜5.2 to 7 kg/L). The amount of microsphere component added to adrilling fluid will depend on the density of the microsphere component,the initial density of the drilling fluid (without a microspherecomponent) and the desired final density of the drilling fluid. Forexample, reducing the density of a 16 lbs/gallon drilling fluid to a 10lbs/gallon would require the addition of about 45 volume percent (orabout 18 weight percent) of a microsphere component having a density ofabout 0.4 g/cm³.

The present invention also provides a method of drilling comprising thestep of circulating a drilling fluid down a drill string and up anannulus between the drill string and bore hole, and introducing amicrosphere component to said drilling fluid in an amount sufficient toreduce the density thereof. The drilling fluid is delivered at asufficient volumetric rate and pressure of effect said circulation downsaid drill string, out a drill bit and up the annular space Themicrosphere component may be added to the drilling fluid at the surfaceand circulated down the drill string and up the annulus of the wellbore. Preferably, the microsphere component is pumped in a fluidvehicle, such as water, and pressure injected into the annulus betweenthe drill string and the well bore to reduce the density of the drillingfluid that has been pumped from the surface down the drill string. Insuch a case, the microsphere component does not come into contact withthe high shear environment of the drill bit. If desired, the microspherecomponent may be injected at multiple points along the annulus from theseabed to the surface.

In the method of the present invention, the pressure of the drillingfluid may be controlled to prevent blowouts, kicks or other uncontrolledpressure conditions. Under most well drilling applications in permeableformations, the drilling fluid pressure should be kept between porepressure of the well and the fracturing pressure of the surround wellformation. If the fluid pressure is too low, the formation fluid canforce the fluid from the well-bore or annulus resulting in a kick orblowout. If the fluid pressure is too high the formation adjacent thewell bore may fracture resulting in loss of fluid circulation and lossof fluid and cuttings to the fracture.

If desired, the method may further include a separation step whereby themicrosphere component is separated from the recovered fluid. Such aseparation step may include a preceding or subsequent step where thedrill cuttings are separated to the recovered fluid. Such a microspherecomponent separation step may include a screening step, where themicrosphere component is screened from both larger and smallercomponents of the recovered fluid. For example, the returning drillingfluid may first be screened to remove cuttings and subsequently screenedto remove the microsphere component. With such a screening step, it ispreferably that the size of the microsphere component be 200 micrometersor more. Alternatively the separation step may comprise a flotation stepwhere the microsphere component is recovered by floating to the surfaceof the recovered fluid due to the low density. As yet anotheralternative, the microsphere component may be separated from therecovered fluid by a centrifugal or cyclonic means whereby the returningdrilling fluid is fed to a hydrocyclone and rapidly spun so that heavierdensity materials, such as cuttings are separated from light components,such as the microsphere, by centrifugal and centripetal forces.

The following examples are provided to illustrates some embodiments ofthe invention and are not intended to limit the scope of the claims. Allpercentages are by weight unless otherwise noted.

EXAMPLES

Glossary

-   A-174 Silane; 3-(trimethoxysilyl)propyl methacrylate, available from    Dow Corning; Midland Mich.-   Adflex™ KS-359; polyproplylene available from Basell, Wilmington,    Del. Blox™ 220; high adhesion thermoplastic epoxy resin, Dow    Chemical Co., Midland, Mich.-   Cumene hydroperoxide; C₆HSC(CH₃)₂OOH; available from Sigma-Aldirch,    Milwaukee, Wis.-   Irgacure™ 651; Methylbenzoylbenzoate, available from Ciba Specialty    Chemicals, Tarrytown, N.Y.-   Lexan 123™; polycarbonate available from General Electric,    Pittsfield, Mass.-   Moplen™; Polypropylene available from Basell, Wilmington, Del.-   SRC 7644™; Polypropylene available from Exxon/Mobil, Edison, N.J.-   SR 351™; Trimethylolpropane triacrylate, available from Sartomer,    Exton, Pa.-   RD 710™; Phenylic resin, available from 3M Company, St. Paul, Minn.    Test Methods    Glass Microsphere Strength Test

An APP strength tester (available from Advanced Pressure Products,Ithaca, N.Y.) was used to determine the collapse strength of themicrosphere component. The sample to be tested was suspended in glyceroland placed in a balloon. The balloon was then inserted into the strengthtester and pressure is applied until the specified percentage ofmicrospheres are ruptured (ASTM D3102-78 with 10% collapse and percentof total volume instead of void volume).

Glass Microsphere Size Measurement Test

The size distribution of each batch of glass microspheres was determinedusing Model 7991-01 Particle Size Analyzer (Leeds and Northrup,Pittsburgh, Pa.).

Glass Microsphere Density Determination Test

A fully automated gas displacement AccuPyc 1330 Pycnometer (availablefrom Micromeritic, Norcross, Ga.) was used to determine the density ofthe glass microspheres according to ASTM D-2840-69.

Preparation of Glass Microspheres

The process that was followed for making glass microspheres isessentially described in U.S. Pat. No. 4,391,646 (Howell; Example 1) andthe composition of the glass used is described in U.S. Pat. No.4,767,726 (Marshall; Example 8). Glass microspheres used to makecomposites typically had a 90% size range of 10 μm-60 μm with a densityof 0.4 g/cm³.

Preparation of Extruded Microsphere Composite

Various thermoplastic materials were co-extruded with glass microspheresusing a 33 mm co-rotating twin screw extruder (Sterling ExtruderCorporation, Plainfield, N.J.) with a length to diameter ratio of 24:1,multiple feed ports fitted with an underwater pelletizer (GalaIndustries, Eagle Rock, Va.). Two volumetric feeders (Accurate DryMaterials Feeder, Whitewater, Wis.) were used to feed additives into theextruder with a screw speed of 250 rpm resulting in a die output rate of5.7 pounds/hr (˜2.6 kg/hr). The material was fed in a polymer/glassmicrospheres weight ratio of 12.7/7.3. The compounding temperature rangefrom hopper to die is cited in Table 1 below. Pelletized material isdried at room temperature for several days before packaging. TABLE 1Strength Strength Strength (@ Temp Density (psi @ (psi @ 19,900 psi;Example Polymer range (° C.) (g/cm³) 10% loss) 20% loss) % loss) 1Blox ™ 220 50-200 0.958 +20,000 +20,000 5.7 2 Moplen ™ 50-220 0.6776,500 9,500 32.7 3 SRC 50-220 0.686 4,750 6.050 41.9 7644 ™ 4 Adflex ™50-220 0.669 1,400 4,800 38.8 KS-359 5 Lexan 50-260 0.912 17,600 +20,00016 123 ™The strength values in Table 1 show that composites of all polymersexhibited suitable strength for drilling applications.

Examples 6-10 Absorbency of Composites to Drilling Fluids

For Examples 6-10 composite microspheres (1.0 g) were placed in drillingfluids (10.0 g; available from Halliburton Energy Services) asidentified in Table 2. The sample was allowed to set at room temperaturefor four days. The mixtures were then filtered through a 250 meshscreen, and the solid composite microsphere material was allowed todrain for 1 hour. The composite microsphere sample was then weighed(w_(f)) and % wt gain was calculated using the formula:${\%\quad{Wt}\quad{Gain}} = {\frac{w_{f} - 1.0}{1.0} \times 100}$

Results are listed in Table 2. TABLE 2 % weight gain of microspherecomposites in various drilling fluids. % Wt % Wt % Wt Gain Gain % WtGain Petrofree Petrofree Gain Example Polymer Petrofree LV SF LVT 200 6Blox ™ 220 6.3 3.0 4.8 3.4 7 Moplen ™ 13.6 15.7 17.8 13.9 8 SRC 764417.7 18.5 22.9 28.6 9 Adflex ™ 55.1 57.5 65.2 93.0 KS-359 10 Lexan ™ 2.04.3 1.7 1.0

Examples 11-13 Preparation of Composite Microspheres with AcrylatePolymers

The composites were prepared as described in U.S. Pat. No. 6,620,214.

Procedure #1: General Procedure for Making a Composite MicrospherePrecursor Slurry

A slurry was prepared by thoroughly mixing glass microspheres, acrylateresin, and initiators, using a mixer (obtained from Hobart Corporation,Troy, Ohio; model number A120T). Specific formulation can be found inTable 3. The abrasive slurry was mixed in the mixer on low speed using aflat-beater style impeller for 30 minutes and heated to a temperaturewithin the range from about 90° F. (32° C.) to about 120° F. (49° C.)due to mechanical heating and heat of reaction. At this point, theabrasive slurry was very thick with cement-like handlingcharacteristics. The mixed slurry was then placed in a refrigerator forat least 45 minutes to cool before further processing. The temperatureof the refrigerator was in the range from about 40° F. (4° C.) to about45° F. (7° C.).

Procedure #2: General Procedure for Making Composite MicrospherePrecursor Particles

The composite microsphere precursor slurry was formed into aggregateprecursor particles with the aid of the “QUADRO COMIL” material formingapparatus (obtained from Quadro Incorporated, Milbourne, N.J. under thetrade designation “QUADRO COMIL”; model number 197). Depending on thedesired cross sectional shape of the composite microsphere precursorparticles, different shaped orifices were used. Conical 10 screens withcircular shaped hole orifices were used to produce composite microsphereprecursor particles with circular shaped cross sections.

The slurry was added to the hopper of the “QUADRO COMIL” by hand whilethe impeller was spinning at a preset speed (rpm) of 350. The rotatingimpeller forced the slurry through the orifices in the conical screenand when a critical length (typically, a critical length is reached whenthe weight of the particle is greater than any adhesive force betweenthe formed composition and the perforated substrate) was reached, thefilamentary shaped composite microsphere precursor particles separatedfrom the outside of the screen, and fell by gravity through a UV curingchamber (obtained from Fusion UV Systems, Gaithersburg, Md.; model #DRE410 Q) equipped with two 600 watt “d” Fusion lamps set at “high” power.The composite microsphere precursor particles were at least partiallycured by exposure to the UV radiation and thereby converted intohandleable and collectable particles.

In some of the examples below the composite microsphere precursorparticles were further at least partially cured by placing the particlesin aluminum pans and at least partially thermally curing them in aforced-air oven (obtained from Lindberg/Blue M Company, Watertown, Wis.;model number POM-246F) for about 5 hours to about 8 hours and at about340° F. (171° C.) to about 360° F. (182° C.). Optionally, the at leastpartially cured composite microsphere precursor particles were reducedin size by passing them through the “QUADRO COMIL”. Typically, particlesare reduced in size by passing them through the “QUADRO COMIL,” with the“QUADRO COMIL” equipped with conical screens that have relatively largerorifices than those used for forming composite microsphere precursorparticles (see examples for specific details). For particle sizereduction, the impeller rotation speed of the “QUADRO COMIL” was set at252 rpm.

Results of Density and Strength testing are listed in Table 4.

Example 14 Preparation of Composite Microspheres with Phenylic Resins

Example 14 was prepared essentially following the procedures describedabove in Preparation of Composite Microspheres with Acrylate Polymerswith the exception that the slurry composition in Procedure #1 isreplaced with the composition cited in Table 5 and the microsphereprecursor particles in Procedure #2 are subjected to temperatures atabout 260° F. (127° C.) to about 280° F. (138° C.) for 10-30 minutesinstead of temperatures at about 340° F. (171° C.) to about 360° F.(182° C.) for 5-8 hours to effect thermal cure.

Results of Density and Strength testing are listed in Table 6. TABLE 3Composition of Acrylate Composite Microsphere Slurries Material Example11 (g) Example 12 (g) Example 13 (g) SR351^(tm) 225 225 275 Cumene 2.52.5 2.5 hydroperoxide Glass microspheres 500 450 400 A-174 45 45 45Irgacure 651 2.5 2.5 2.5

TABLE 4 Densities and Strengths of Acrylate Composite Microspheres.Strength Strength Example Density (g/cm³) (psi @ 10% loss) (psi @ 20%loss) 11 0.5359 5200 6400 12 0.5315 4550 5600 13 0.6109 8650 11,300

TABLE 5 Composition of Phenolic Composite Microsphere Slurry MaterialExample 14 (g) Glass microspheres 425.0 710 phenolic resin 225.0 DIwater 30.0

TABLE 6 Densities and Strengths of Phenolic Composite Microspheres.Strength Strength Example Density (g/cm³) (psi @ 10% loss) (psi @ 20%loss) 14 0.5332 5800 10,850

1. A drilling fluid composition comprising a fluid component, aviscosifying component and a composite microsphere component in anamount sufficient to reduce the density of the composition, wherein saidcomposite microsphere component comprise pellets comprising a continuousphase of polymeric resin binder and microspheres dispersed therein. 2.The composition of claim 1 wherein the size of said pellets is from 200to 4000 micrometer s.
 3. The composition of claim 1 wherein the densityof said pellets is from 0.4 to 1.0 g/cm³.
 4. The composition of claim 1wherein said pellets comprise 20 to 75 wt. % microspheres.
 5. Thecomposition of claim 1 wherein said pellets comprise 20 to 60 wt. %microspheres.
 6. The composition of claim 1 wherein said microspheres ofsaid composite microsphere component comprises glass, ceramic orpolymeric hollow microspheres.
 7. The composition of claim 6, whereinthe density of said microspheres is from 0.1 to 0.9 g/cm³.
 8. Thecomposition of claim 6, wherein the density of said microspheres is from0.2 to 0.7 g/cm³.
 9. The composition of claim 1 wherein said compositemicrosphere component is in amounts sufficient to reduce the density ofsaid composition to 8 to 13 pounds/gallon (5.2 to 7.5 kg/L).
 10. Thecomposition of claim 6 wherein said microspheres are unitary glassmicrospheres.
 11. The composition of claim 6 wherein the density of saidglass microspheres is from about 0.2 to 0.7 g/cm³.
 12. The compositionof claim 6 wherein said unitary glass microspheres are from 5 to 1000micrometers in diameter.
 13. The composition of claim 6 wherein saidunitary glass microspheres are from 100 to 1000 micrometers in diameter.14. The composition of claim 1 wherein the composite microspherecomponent is at least 200 micrometers in size.
 15. The composition ofclaim 1 wherein said resin binder is a thermoplastic or thermoset resin.16. The composition of claim 15 wherein said thermoplastic is selectedfrom polyolefin homo- and copolymers; styrene copolymers andterpolymers; ionomers; ethyl vinyl acetate homo- and copolymers;polyvinylbutyrate homo- and copolymers; polyvinyl chloride homo- andcopolymers; metallocene polyolefins; poly(alpha olefins) homo- andcopolymers; ethylene-propylene-diene terpolymers; fluorocarbonelastomers; polyester polymers and copolymers; polyamide polymers andcopolymers, polyurethane polymers and copolymers; polycarbonate polymersand copolymers; polyketones; and polyureas; and blends thereof.
 17. Thecomposition of claim 15 wherein said thermoset resin is selected fromepoxy resins, acrylated urethane resins, acrylated epoxy resins,ethylenically unsaturated resins, aminoplast resins, isocyanurateresins, phenylic resins, vinyl ester resins, vinyl ether resins,urethane resins, cashew nut shell resins, napthalinic phenylic resins,epoxy modified phenylic resins, silicone resins, polyimide resins, ureaformaldehyde resins, methylene dianiline resins, methylpyrrolidinoneresins, acrylate and methacrylate resins, isocyanate resins, unsaturatedpolyester resins, and blends thereof.
 18. The composition of claim 1,wherein the collapse strength of said composite microsphere component is4000 psi (27.6 MPa) or greater.
 19. The composition of claim 1 whereinthe composite microsphere component comprises from 25 to 50 volumepercent of said drilling fluid composition.
 20. The composition of claim1, wherein said viscosifying agent is selected from clays, starch,carboxymethylcellulose, natural gums or synthetic resins, andcombinations thereof.